Putting words in glappkaeft's mouth, or fingers, he's saying that the shell theorem does not say that gravitational force decreases with increasing depth. He's correct. The shell theorem does not say that. You have to add the assumption of a uniform density to reach that conclusion. He's also correct in that gravitational force inside the Earth reaches a maximum value at the core/mantle boundary. The gravitational force halfway down to the center of the Earth is about 9% higher than the surface value. This is because the Earth's core comprises a bit less than 1/3 of the Earth's total mass but occupies a bit more than 1/6 it's total volume.
What the shell theorem does say is that for an object with a spherical mass distribution (density is a function of radial distance from the center), it's only the mass below that counts. You can use the shell theorem to find the condition that make gravitational force increase or decrease with increasing depth. Defining ##\rho(r)## as the density at some distance ##r## from the center and ##\bar{\rho}(r)## as the average density of all the stuff below that distance, you should find that gravitational force increases with depth if ##\rho(r) < \frac 2 3 \bar{\rho}(r)##.